I'm cursing @ Monk of Zamorak, and when it's supposed to say "Your opponents attack has already been weakened." it says "Someone else is already fighting that."
kthxbai
This seems to be fixed. However part of your bug report is related to another bug, with the weakening spells not actually weakening the enemy.
Moving to the fixed section as another bug report is in place for the aforementioned issue.
- Dev Sean